Integrated circuits (ICs) are used in a wide variety of electronic equipment including everything from consumer products to space probes. In the vast majority of applications, primary design constraints usually include power dissipation, heat generation, cost, and size. ICs are particularly useful because they can provide millions of transistors in a vary small package. Each transistor, however, dissipates a certain amount of power and generates a certain amount of heat. The potentially millions of transistors in an IC can consume a relatively large amount of power and dissipate a relatively large amount of heat.
In order to conserve power and reduce heat problems in ICs, core IC logic is often designed to operate at relatively low voltage levels. The low internal operating voltages of core logic are often below the normal operating voltages needed by other components. So, for instance, if ICs and other components are used together in electronic equipment, internal logic in an IC may operate at 1.8 volts, but IC input ports may experience signals from external logic of up to 5 volts.
FIG. 1 illustrates one embodiment of a prior art input buffer to convert input signals from external logic 110 to an operating voltage for core logic 120 within IC 130. Transistors 150 and 160 are complementary metal-oxide-semiconductor (CMOS) transistors. When external logic 110 asserts a high voltage, transistor 160 turns on and transistor 150 turns off to couple core logic 120 to ground. When external logic 110 asserts a low voltage, transistor 150 turns on and transistor 160 turns off to couple core logic 120 to core operating voltage source 140. This type of circuit cannot handle high voltage on the gate directly.
If the magnitude of the voltage asserted on the gates of transistors 150 and 160 becomes too large, electric current may arc across the transistors permanently damaging the oxides within the transistors and rendering the transistors inoperative. The maximum voltage that a transistor can withstand across the oxide, either from the gate to the source or the gate to the drain, is process dependent. That is, depending on how thick the oxide is and the profile depth of gate-source junctions and gate-drain junctions for a particular transistor technology or manufacturing process, the maximum tolerable voltage varies.
Because size is often a primary design constraint, with each new generation of integrated circuit technology, transistors within ICs, and hence oxide thicknesses, tend to become smaller and the number of transistors per chip tends to increase. Increased numbers of transistors translates into lower desired operating voltages to reduce heat and power dissipation. Thinner oxides also translate into lower maximum tolerable voltage levels. In which case, with each new generation of IC technology, the difference between internal operating voltages and external operating voltages tends to increase, but the maximum tolerable voltage drop across individual transistor oxides tends to decrease.
Continuing the above example, if the process dependent maximum tolerable voltage for transistors comprising core logic 120 in FIG. 1 was less than the voltage asserted by external logic 110, transistors 150 and 160 would have to be designed to withstand higher voltages than internal logic 120. That is, in order take advantage of an IC technology that uses very small transistors in core logic 120, transistors 150 and 160 would need to be created using a different, higher voltage technology than the internal logic in order to prevent core logic 120 from being permanently damaged by large input voltages.
Unfortunately, each additional technology on an IC tends to add cost. For instance, additional technologies usually require additional, and costly, processing steps to make different oxide thicknesses or carrier electron densities. Additional technologies also tend to make ICs more difficult and costly to design because optimizing performance for one technology may interfere with the performance of other technologies. High voltage technologies, which tend to rely on thicker oxides, also tend to operate slower and dissipate larger amounts of energy and heat.
Therefore, for at least the reasons discussed above, a need exists for an improved integrated circuit input buffer which can withstand a higher input voltage than internal logic, but does not require the addition of a higher voltage transistor technology.